This study will implement, evaluate, and optimize a neuroprosthesis based on functional neuromuscular stimulation (FNS) which will restore shoulder function to spinal cord injured individuals with C5 tetraplegia. Such individuals retain little or no voluntary control over motions acting to move the upper arm toward the midline, due primarily to paralysis of the pectoralis major (PM) and latissimus dorsi (LD) muscles. This loss of control significantly reduces the range of motion of the hand, excluding an important workspace volume near the midline, and prevents arm stabilization in the natural abducted postures used in many tasks like eating and writing. Restoration of these functions would significantly improve the independence of these individuals in a number of daily activities, improving their quality of life and reducing their attendant care costs. The main hypothesis of this study is that FNS of a small number of sites in the PM and LD muscles can significantly improve shoulder function in horizontal flexion, adduction, and internal rotation in individuals with C5 tetraplegia. The stimulated contractions will restore the lost motions, while retained voluntary control of antagonistic muscles will be used by the individual to overcome the stimulated contractions and achieve intermediate positions and external forces in a completely natural manner. After verifying the hypothesis that individuals with a C5 Function level retain sufficient stimulated strength in the PM and LD and adequate voluntary control over their antagonists, experiments will test the hypotheses that the shoulder neuroprosthesis expands the workspace volume accessible to the hand, improves postural stability, and allows more rapid and accurate arm movements. Several general methods for improving control of the partially paralyzed shoulder will also be developed. Shoulder stiffness properties will be used to identify deficits in postural stability in a systematic manner and to suggest changes in electrical stimulation patterns to correct the deficits. The feasibility of using electromyographic recordings from voluntarily-controlled shoulder muscles to modulate stimulation of the paralyzed muscles will be investigated, since such modulation could improve movement performance, prevent fatigue, and compensate for changes in contraction strength in different shoulder positions.